The present invention relates generally to integrated circuit (IC) fabrication. More particularly, the present invention relates to a process for improving the etch stability of ultra-thin photoresist utilized in IC fabrication.
The semiconductor or IC industry aims to manufacture integrated circuits (ICs) with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features. The ability to reduce the size of structures, such as, gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance.
With conventional lithography systems, radiation is provided through or reflected off a mask or reticle to form an image on a semiconductor wafer. Generally, the image is focused on the wafer to expose and pattern a layer of material, such as, photoresist material. In turn, the photoresist material is utilized to define doping regions, deposition regions, etching regions, or other structures associated with integrated circuits (ICs) to one or more layers of the semiconductor wafer. The photoresist material can also define conductive lines or conductive pads associated with metal layers of an integrated circuit. Further, the photoresist material can define isolation regions, transistor gates, or other transistor structures and elements.
Presently, lithography systems are typically configured to expose the photoresist material at a radiation having a wavelength of 248 nanometers (nm). However, because the resolution of features is, in part, proportional to the inverse of the exposure wavelength, it is desirable to pattern photoresist material using radiation at shorter exposure wavelengths (e.g., 193 nm, 157 nm, 126 nm, or 13.4 nm). Unfortunately, materials, equipment, and/or fabrication techniques suitable for 248 nm lithography do not provide similar results at the shorter exposure wavelengths. Moreover, few, if any, materials or processes tailored for use with shorter exposure wavelengths exist.
One of the problems associated with the use of organic-based photoresist materials conventionally used in 365 nm or 248 nm lithography is the high optical absorption per unit thickness at the shorter wavelength lithographic radiation. Conventional photoresist materials become increasingly opaque at the shorter wavelengths and the necessary photochemical change will not occur throughout the entire thickness of the photoresist material.
To overcome this drawback, a thinner layer of photoresist material is used for the shorter lithographic wavelengths. A standard or conventional thickness of photoresist material for 248 nm lithography is more than 0.5 xcexcm. The 248 nm photoresist materials are typically based on phenolic polymers. For 193 nm lithography, 193 nm photoresist materials based on acrylite and/or alicyclic polymers may be provided at a thickness of 0.4 to 0.3 xcexcm. For even shorter lithographic wavelengths or to further enhance 193 nm lithography, ultra-thin resists (UTRs) are used, which are conventional photoresist materials provided at a thickness of less than 0.25 xcexcm.
A certain amount of photoresist material (e.g., vertical thickness) is typically consumed during IC fabrication processes, e.g., resist trimming and/or etch processes. Unfortunately, when a thinner layer of photoresist material is used, such as, photoresists in 193 nm application or ultra-thin photoresists, there may not be enough photoresist material remaining after consumption to maintain pattern integrity, survive subsequent processes, and/or with which to successfully transfer the pattern to underlying layers. In other words, thinner layers of photoresist material suffer from the disadvantage of low or insufficient etch stability.
Thus, there is a need for a process for effectively extending the use of conventional photoresist materials for shorter lithographic applications in the vacuum ultraviolet (VUV), deep ultraviolet (DUV), or extreme ultraviolet (EUV) wavelength range. There is a further need for a process for increasing the etch stability of photoresists used in 193 nm application or ultra-thin photoresists.
One exemplary embodiment relates to a method of increasing an etch stability of a photoresist layer. The method includes providing the photoresist layer at a thickness less than 0.25 xcexcm, for use in vacuum ultraviolet lithography, deep ultraviolet lithography, or extreme ultraviolet lithography. The method further includes exposing the photoresist layer to a plasma, and transforming the exposed surfaces to form a hardened shell. The photoresist layer includes exposed surfaces, and the hardened shell increases the etch stability of the photoresist layer.
Another exemplary embodiment relates to an integrated circuit fabrication process. The process includes patterning a feature on a photoresist layer disposed over a substrate. The feature is patterned in accordance with a pattern provided on a mask or reticle and a radiation at a deep ultraviolet or extreme ultraviolet lithographic wavelength. The process further includes developing the photoresist layer, and exposing the photoresist layer to a plasma. The process still further includes transforming the top surface and the side surfaces to form a hardened surface, and etching the substrate in accordance with the transformed feature. The patterned photoresist layer includes at least one feature having a top surface and side surfaces. The exposing step occurs after the developing step and before the etching step. An etch stability of the feature is a function of the hardened surface.
Still another exemplary embodiment relates to a feature patterned on a photoresist layer disposed over a semiconductor substrate. The feature includes exposed surfaces, and an untreated region enclosed by the exposed surfaces. The exposed surfaces are structurally denser than the untreated region due to at least one of a fluorination, an ion implantation, and an electron beam curing. The feature is lithographically patterned using at least one of a deep ultraviolet lithographic wavelength, a vacuum ultraviolet lithographic wavelength, and an extreme ultraviolet lithographic wavelength or has a vertical thickness less than approximately 0.25 xcexcm.